In a small (1- to 2-quart) saucepan, combine the sugar, water and pasilla chile pieces. Set over medium-high heat and bring to a simmer, stirring constantly to dissolve the sugar granules. Remove from the heat, cool and store in a closed container.
In a cocktail mixing glass, add ice cubes, mezcal, pasilla syrup and orange bitters and stir to to combine. Pour over ice into an Old Fashioned glass and serve immediately.